This invention relates to systems and methods for determining whether a cable network is compliant with a set of standards for two-way data communication.
In cable network systems (e.g., a hybrid fiber-coaxial (or HFC) plant), digital data is carried over radio frequency (RF) carrier signals. At the interfaces of a cable network are cable modems. These devices modulate digital data for “upstream” transmission on a broadband media and demodulate modulated RF signals for “downstream” reception of digital data.
Cable networks are composed of passive and active devices, each having a particular attenuation and frequency response. The overall frequency and amplitude response of a cable network system varies nearly continually. Cable lines are installed in new geographic areas, new cable modems and components are installed on existing segments of the network, the condition of existing lines deteriorates or improves, the performance of amplifiers, splitters, etc. changes, and so on. If the condition of the overall cable network or some subsection of it deteriorates below certain physical margins, the system may become partially or wholly unusable. Because cable plants are large, complex, and dynamic, it is very difficult to model or predict their performance.
Currently, cable networks are tested for compliance with particular two-way communication standards before cable modems are brought on-line. The testing typically involves defining an “X” point at or near the network's “headend” and then monitoring the frequency and power of signals received at that point. To evaluate the behavior of the entire network, signals are transmitted from numerous points on the cable network. Testing may be accomplished by putting a frequency analyzer at point “X” and then causing test signals to be generated at various locations on the network. These test signals may be provided by a specialized piece of apparatus that connects to the system at particular locations. The apparatus may generate test signals at, for example, 45 dBmV scanned over a frequency range of 5-42 MHz, a standard frequency range for upstream data communications in cable networks. The spectrum analyzer at point “X” records the amplitude versus power spectrum it receives from the source of the signal. This gives some indication of the frequency response of the cable network from the point where the signal is generated. A technician typically moves the signal generating apparatus from point-to-point on the network so that the frequency response of the network can be mapped for various positions over the network. If a problem is discovered, the technician may attempt to correct it by installing an equalizer at an appropriate location, for example.
A cable network suitable for two-way data communications should have a fairly linear response (increases in transmitted power cause a proportional increase in measured power at the X point). Such network should also have a relatively flat frequency response (the detected power level is relatively uniform over the frequency range being tested). Further, a compliant system should have relatively low noise levels. In specific approaches, compliance with these criteria may be determined by comparison of the network response against a threshold peak to valley (PtV) value across the upstream frequency range, a threshold carrier to noise (C/N) ratio, and/or an amplitude response factor.
One major drawback to the current method of testing a cable network for compliance is that the testing cannot be conducted while live two-way communication is taking place. Or at least the testing cannot be conducted over the entire upstream frequency range. At any given time during live communication, a particular frequency band is used for upstream communications. If the testing takes place in this band, the live data may become unreadable. As a result, complete testing, across the entire upstream frequency range, cannot be conducted while the cable network is in service. Of course, the cable service can be temporarily halted, but this results in a loss of revenue to the cable company and associated service providers.
Because complete testing cannot be easily conducted while a cable network is in service, the network may drift toward marginal performance and even become partially or wholly unusable without forewarning. Thus, it would be desirable to have an improved testing method and/or system that allowed complete cable network tests to be conducted without disrupting the cable service.